Q: macbook pro shuts itself down

My Macbook Pro, which is a 2010 version running Yosemite 10.10.4 often just shuts itself down and re-starts for no reason obvious to me.  When I look at the fault message, it often refers to CPU panic, or similar.  I notice that it most frequently happens when using MAIL, but not only then.  Can anyone suggest a fix for it?  Oh, and I have repaired the disk and Disk permissions, so i don't think its that.

Thanks!